Microtomes and ultramicrotomes are, as a rule, equipped with two knives. One knife serves to trim the sample, and the other knife serves to produce the thin sections. Microtomes and/or ultramicrotomes can also be provided with a cooling chamber. During operation of a microtome with a cooling chamber, two or three knives are usually present in the chamber. One is used for trimming, and the second for producing the ultrathin sections. When a new knife is introduced from outside into the cooling chamber, it is necessary to wait a few minutes for temperature stabilization.
In the case of the CR-X chamber of RMC, for a knife change the entire chamber must be displaced laterally by means of a drive. This displacement unit is located at the point on the ultramicrotome at which the chamber is attached. If a base-mounted illumination system is built into the chamber as a presetting aid for the alignment operation between knife and specimen, it is also displaced laterally. The illumination geometry must, however, be maintained exactly while observing the presetting operation with the stereomicroscope. The arrangement of the illumination system can therefore only be a compromise between the two knife positions.
The Leica FCS cooling chamber possesses a capability for displacement of the knife holder in the chamber without modifying the position of the illumination system. In this arrangement, it is possible always to position the knife above the illumination system. This additional displacement capability has, however, the disadvantage that operator errors occur. The knife holder is often displaced along with the illumination system for a knife change, thus disadvantageously modifying the illumination geometry.
During operation of a microtome or ultramicrotome, it is also necessary at regular intervals to replace, after a certain operating time, the knives that are being used. When a cooling chamber is used, operation of the microtome is interrupted during replacement of the knife, since the temperature equilibrium (or “climate”) inside the cooling chamber is disturbed by the introduction of the new knife. In addition, it is once again necessary to position, i.e. align, the specimen that is to be cut with respect to the new knife, in accurately positioned and exact fashion. This so-called presetting operation also requires time, and care must be taken that neither the knife nor the specimen is damaged in the process.